Volume 21, Number 12—December 2015
Probability of Spirochete Borrelia miyamotoi Transmission from Ticks to Humans
To the Editor: Borreliosis caused by Borrelia miyamotoi is an emerging disease transmitted by Ixodes ticks (1). Each year in the Netherlands during 2007–2009, ≈70,000 bites by hard ticks occurred per 1 million inhabitants (2). In the Republic of Udmurtia, Russia, ≈10,000 hard tick bites per 1 million inhabitants are reported annually among persons seeking medical help. Recent studies indicate that almost 3% of I. ricinus ticks in the Netherlands and 2%–6% of I. persulcatus ticks in Russia are infected with B. miyamotoi (1,3,4). Human exposure is substantial, and comparable exposure to B. miyamotoi is expected in many Eurasian countries and in North America (4,5). The probability of B. miyamotoi transmission from ticks to humans requires examination to estimate the true risk for human disease.
In Izhevsk (population 650,000), a city in European Russia (Republic of Udmurtia), we identified 95 human cases of B. miyamotoi infection during 2010–2014 (6). In this city, primarily because of concern about tickborne encephalitis (TBE), all patients with suspected tickborne infection are hospitalized in the Republican Hospital of Infectious Diseases (RHID). A service also enables tick-bitten persons to bring the removed tick for PCR for TBE virus (TBEV) and B. burgdorferi sensu lato. We supplemented that with PCR testing for B. miyamotoi (3).
In June 2014, twenty-four persons (≈5% of those bitten by ticks subjected to PCR-based investigation for TBEV, B. burgdorferi sensu lato, and B. miyamotoi) were bitten by adult I. persulcatus ticks infected with B. miyamotoi only. We informed these persons of their results and about the clinical features of B. miyamotoi infection and recommended self-observation during 2 months (twice the maximum incubation period for B. miyamotoi infection [3,6]). These persons were advised to contact their medical supervisor at RHID (D.S. Sarksyan) if fever, fatigue, erythema migrans, or any other possible symptom of a tickborne infection developed. In 2 patients, such symptoms developed: one 12 days (patient 1), the other 16 days (patient 2), after the tick bite. B. miyamotoi DNA was detected by PCR in their blood on admission to RHID. Neither IgM nor IgG were found by a nonspecific ELISA (Omnix, St. Petersburg, Russia ) that reacts with serum from B. burgdorferi sensu lato–infected and B. miyamotoi–infected persons. However, Borrelia IgM and IgG were detected in serum obtained 12 and 45 days after illness onset from patient 1 and 6 and 39 days later from patient 2. Clinical characteristics were typical of B. miyamotoi infection: chills, sweating, headache, dizziness, fatigue, thirst, nausea, vomiting, fever (axillar temperature 38.8°C in patient 1 and 39.0°C in patient 2). Erythema migrans was absent. Both patients had modest thrombocytopenia (134 [patient 1] and 118 [patient 2] × 109 platelets/mL [reference range 150–400 × 109 platelets/mL]) and increased band neutrophils (10% [patient 1] and 8% [patient 2] of leukocytes [reference range 1%–5%]). Patient 2 had clinical and laboratory signs of kidney dysfunction (oliguria, leukocytes, erythrocytes, and epithelial cells in a urine sample), a complication observed in 18% of 95 patients with B. miyamotoi disease (4). Both patients were treated with doxycycline (100 mg 2×/d) for 10 days; they clinically recovered, and laboratory abnormalities returned to reference ranges at discharge 12 days after admission.
The remaining 22 persons did not report any malaise and were examined 1 month after tick bite. They appeared healthy at that time, and PCR and ELISA gave negative results, arguing against possible asymptomatic B. miyamotoi infection.
We estimated the probability of B. miyamotoi transmission to humans to be 8.3% (95% CI 4%–18% using a Bayesian approach  or 95% CI 0%–21% using an SPSS bootstrapping procedure [SPSS Inc., Chicago, IL, USA]). For comparison, among 68 persons bitten by B. burgdorferi sensu lato–infected ticks in the Netherlands, erythema migrans developed in 4.4% (95% C.I. 2.1%–8.3%) persons; 3 (4.4%) others seroconverted without clinical symptoms (9).
This pilot study has several limitations. We did not follow up persons bitten by B. burgdorferi sensu lato– or TBEV-infected ticks because they received either antimicrobial drugs or anti-TBE immunoglobulin as a preventive measure. Because of labor constrains, we did not study persons bitten by “PCR-uninfected” ticks; however, they were not hospitalized at RHID, the only hospital in the region for patients with evident tick-borne diseases. We did not use any serologic techniques specific for relapsing fever Borreli (e.g., GlpQ ELISA). Although we did not test for Rickettsia or Babesia spp., we did not find TBEV RNA, B. burgdorferi sensu lato 16S RNA, pathogenic Ehrlichia 16S RNA, or pathogenic Anaplasma DNA in the 2 B. miyamotoi–positive patients’ blood samples.
We demonstrated that the transmission rate of B. miyamotoi appears to be equal to, or higher, than that of B. burgdorferi sensu lato (1,9,10). Our data indicate that, annually, clinical B. miyamotoi infection might develop in at least 0.005% of persons living in regions to which Ixodes spp. ticks and B. miyamotoi are endemic. This estimate corresponds to ≈33 cases annually in Izhevsk, which is similar to the previously published results of hospital-based surveillance for B. miyamotoi (3,6).
This study was supported by the grant of Russian Scientific Foundation (project no. 15-15-00072) to A.E.P., D.S.S., and L.S.K.
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